1. Field of the Invention
This invention relates to a dry-etching microfabrication processes and more particularly to a topography simulator for the dry-etching microfabrication processes for determining the optimum conditions of dry-etching on a substrate
2. Description of the Prior Art
The state-of-art semiconductor technology as seen in fabrication of dynamic random access memories (DRAMs) has been advanced to further enhancement of high density and microfabrication technology. The sizes of transistor gates, wirings and capacitor forming elements and so forth are on the half micron range. To realize such sub-micron patternings, dry etching is carried out on underlying materials to be worked by utilizing ions and radicals (active neutrals in excited state) produced in a discharge plasma area, while a resist patterned according to design data is used as mask.
FIG. 1 shows a typical parallel plate dry etcher of the reactive ion etching type (RIE) and related module configurations of a simulational system. A wafer spacemen 10 is placed on a cathode electrode 11 supplied with RF power via a coupling condenser 12. Ions are accelerated substantially in a vertical direction and induced onto the spacemen 10, while traveling through a bulk plasma region and a sheath region formed over a cathode electrode 13. Radicals, on the other hand, are transported to the wafer surface in isotropic kinetic direction.
FIG. 2 shows four kinds of surface reactions which determine etching profiles. A reaction scheme of deposition is also shown in FIG. 2 where a compound layer is deposited on the substrate when non-reactive radicals are coming. The first surface reaction is a thermally induced chemical reaction as seen in FIG. 2(a). Active radicals are adsorbed on the substrate surface, and then these radicals react with substrate material by the thermal energy of the substrate to produce reaction products having a high vapor pressure. The reaction products are desorpted so that the substrate is etched isotopically. The second and third are ion sputtering. There are two types of ion sputterings. One is physical sputtering as seen in FIG. 2(b), which is induced by an energetic ion. Atoms of the substrate material obtain kinetic energy by momentum transfer due to collision-cascade mechanism and then are sputtered. As a result, the materials present on the substrate are etched away in the incident direction of the ions. The other is chemical sputtering as seen in FIG. 2(c). Incoming ions of active species induce a chemical reaction with the substrate material. Like the physical sputtering, this reaction etches the substrate material in the incident direction of the ions.
The fourth reaction as shown in FIG. 2(d) is ion-assisted etching. This is a reaction between adsorbed active radicals and substrate material enhanced by a simultaneous ion bombardment. The ion-assisted etching proceeds as follows.
Assume that absorbed layer of active neutrals has been formed on the substrate. Kinetic energy of the incoming ions activates a plurality of the absorbed particles near the impingement of the ions and promotes remarkably chemical reaction between the activated active absorbed radicals and the materials on the substrate. The reaction products are thus desorpted.
Should ions of the same kinetic energy and same mass be impinged, an etching rate obtained when the surface material is exposed to both active radicals and energetic ions simultaneously, is about ten times higher than that of physical sputtering. The ion-assisted etching plays the most important and dominating role among the etching reactions taking ion impingement into account. Similarly, this reaction etches away the materials on the substrate substantially in the incident direction of the ions.
To form minute patternings through etching, there are two criteria to meet, i.e., topography control and selectivity. The first requirement of topography control is anisotropy of etching by which patterns are formed in strict compliance with the sizes of masks and topographic profiles are realized in a vertical or, if desired, tapered orientation. The second requirement of selectivity is that there be enough allowance of etching between the etching rate of mask material and underlying materials and the etching rate of the material to be etched.
As theoretical analysis means for solving the above discussed problems, several topography simulators have been reported. With the development of such topography simulators, various types of reaction processes have been incorporated to simulate fundamental facts of experiments. However, the conventional simulators were designed to give the etching rate and deposition rate defining topographic deformation, as a function of the fluxes of incoming ions and radicals at respective points of the substrate surface, merely through the semi-experimental techniques. In other words, the surface conditions such as the presence of absorbed particles on the substrate were not taken into consideration.
For the conventional simulator, it was, therefore, difficult to evaluate difference between transport rate limited and reaction rate limited in the thermally induced chemical reaction by the active radicals, the ion-assisted etching reaction or the side-wall protection phenomena by competitive process between etching and deposition and so forth. In other words, the conventional simulators were not successful tools which provide satisfactory solution to the above problems in the state-of-art dry-etching processes requiring sub-micron patternings.
Returning to FIG. 1, the simulator consists of three modules. The first module 15 is a particle transport model which calculates the fluxes of radicals and ions arriving at the substrate surface. Ions are accelerated by the electrical field in the sheath region. An angular distribution of ion flux arriving at the substrate surface is approximated by Gaussian distribution. The standard deviation of Gaussian distribution is obtained by the Monte-Carlo technique. Indirect radical flux by re-emission process is included by taking into account the substrate surface profile. The second module 16 is a surface reaction model which calculates etching and/or deposition rates on the substrate surface. This part is the main subject of the present invention. The third module 17 is a string model which expresses the time evolutions of the surface profile.